Chromium plating



Patented Jan. '15, 1946. v

Richard M. Wick, Allentown, Pa.

No Drawing. Application July a. 1939,

7 Serial No. 283,398

10 Claims.

(Granted under the m of lump s, was, as

amended April 30,1928: 370 O. G. 75'!) My invention relates to the electrodeposition of chromium. It pertains more particularly to chromium plating, although it may be successfully utilized in chromium recovery.

My invention comprises a new type of electrodeposited chromium, particularly a new type of chromium coated article, wherein the chromium is hard, homogeneous, malleable, and capable of receiving impacts of sumcient intensity to effect permanent distortion of the coated article without cracking or spelling of the chromium My invention also comprises methods for controlling the conditionsof electrodepositionf to obtain chromium of the type just indicated, and to obtain other results of great value. For example, by my control of the conditions of electrodeposition it is possible to deposit the chromium very rapidly and to obtain coatings of any desired thickness in a very short time as compared with the prior practices.

It is old, of course, to produce'chromium coated article by electrodeposition. The prior art, however, has been directed principally to the application of very thin coatings which are utilized as non-tarnishing surface films. Such coatings, when deposited on a suitable surface such as nickel plate, having a thickness of about 0.00002 inch, have been found sufllcient to maintain surface brightness over a considerable period of time. Such prior art chromium coatings have been obtained by electrodepositing on a suitable base in a solution containing chromic acid and sulphuric acid, the usual content of the sulphuric acid being about 1% of the chromic acid. Ordinarily, a temperature of 45 C. and a current if a more rapid plating process had been available. .11, under the conditions of prior chromium plating, it had been attempted to increase substantially the rate of deposition by substantially increasing the current density, a so-called burnt" chromium deposit would have been formed which is too fragile, and otherwise entirely unfit for mechanical use. Obviously, the prior art methods have not been well adapted to the production of heavy co'atings of chromium because of the slowness with which the chromium has been deposited. 1

density of 150 amperes per square foot of cathode surface have beenused. Under these conditions it is possible to obtain the desired thin coating in from two to four minutes of plating time. Where a heavier coating for wear resistance or for use in machine elements was desired, the practice has been to continue the plating for a longer period of time. quired an exceedingly long time of immersion in the bath, and produced a chromium coating of variable quality because of difficulty in achieving adequate control of the process. The slow rate, involving a long time of deposition, increases the likelihood of the .development of defects in the plated metal during the process, or variations in quality which arise in variations in temperature, current supply, contamination of solution, etc. Then too the equipment necessary to produce a given amount of work had to be far greater than The slow rate of plating re-v The chromium deposit produced by the prior art processes has been inferior in many respects, particularly where the product produced was to be subjected to severe mechanical stresses, the chromium film of coated articles being brittle, and the coating tending to crack and spall particularly when subjected to more or less severe impact.

My invention avoids these defects of the prior art, the chromium deposited i bright, non-tarnishing, and has a high'degree .of malleability. Ferrousarticles for example, coated in accordance with my invention, may be subjected to severe impact, severe enough, in fact, to effect permanent distortion of the ferrous metal, without the chromium coating spelling.

By my methods the chromium may be rapidly deposited, far more rapidly than with the prior art, and coatings far thicker than'those of the prior art produced in a short period of time and with considerable less cost of equipment than that i of the prior art for deposition of equivalent amounts ofchromium. By my process the rate in four minutes or less, depending on the current carrying capacity of the object being plated, and hence on the currentdensity employed for the plating, it is apparent that large scale produc-,

tion of such new articles of commerce becomes feasible, particularly as my processes may be "easily applied to articles which are being con tinuously fed through the plating bath.

Animportant feature of my invention is the use of high current densities, 700 ampe es D square foot of cathode surface, or greater. Ordinarily for chromium plating I employ current densities up to 5000 amperes per square foot of cathode surface. between 800 and 3000 amperes per square foot of cathode surface being the most commonly employed. Usually; for plating purposes, I do notgo above 5000 amperes per square foot of cathode surface because of the unavoidable expense involved in equipment where such great electric currents must be transmitted. However, apart from economic aspects there is no obstacle to employing much higher current densities. For the recovery or winning of chromium from solution, it is entirely feasible to use current densities of the order of 10,000 amperes per square foot of cathode surface, or even higher.

By the use of these high current densities, 700 amperes per square foot of cathode surface, and higher, it is possible to deposit the chromium at a rapid rate and to secure thick deposits in relatively short periods of time, as it has already been indicated above. The wide range of current density at which my process is operable i very important because of the uniformity of properties of the metal which is deposited. This is due to the fact that the process having a wide range of current densities greatly lessens the effect on uniformity of variations in the current density which may occur during any particular operation.

A very important feature of my invention is in the temperatures of the electrolytes used. I have found that to secure my new type of chromium deposit and to successfully use high current densities it is necessary to employ higher electrolyte temperatures than heretofore used. When the temperature is too low it is impossible to use current densities above a very moderate amount. By increasing the working temperature of the electrolyte higher and higher current densities may be employed. If high current densities are applied while the electrolyte temperature is low a very unsatisfactory deposit is produced. For my purposes the temperature of the electrolyte should be above 55 C. At 55 C. the range of current densities employable to get a good deposit is rather narrow, but as the temperatures are elevated above 55 C. the range of available current densities rapidly increases to include higher and higher current densities. For most purposes in carrying out my invention the temperature selected will be above 55 C., but not above 90 C. However, for exceedingly high current densities, say of the order of 10,000 amperes per square foot of surface, the actual temperature may range above 90 C. This is because of the local heating effect in the immediate environment of the cathode area caused by the congestion of current, as

may be clearly seen in the case of a cylindrical cathode in circuit with an anode of larger diameter. .In such case the bath may be cooled to below 90 C. to achieve a thermal balance with the zone of plating to obtain a satisfactory deposit. The

I have found that lead and lead alloys such'as 6% antimonial-lead are suitable for use as anode material at the high current densities I employ. The deterioration of the anode is not accelerated by the greatly increased currents used in my process, probably because the oxide of lead formed at high current densities, which I have observed to be black in the case of the antimonial lead. acts as a protective him so that the anode does not deteriorate with increased velocity on continuous electrolysis.

I employelectrolytes which are qualitatively similar to those of the prior art, in that my electrolytes contain a compound of chromium from which the chromium is to be separated for deposition electrolytically, and a catalyst. Like the prior art, I employ electrolytes containing chromic acid and a compound having the sulphate radical, commonly using sulphuric acid. Quantitatively, however, my baths are different from the prior art baths. i

I usually employ chromic acid between 150 and 600 grams per liter of electrolyte. Preferably I employ from 250 to 500 grams per liter. An exceedingly important feature of my invention is the concentration of the sulphate radical. I have found it important to employ the sulphate radical,

- speciflcallysulphuric acid, in an amount equal to from 3 to 10 grams of the sulphate radical per liter of electrolyte. I have found it important to employ the sulphate radical, specifically sulphuric acid, in an amount of at least 3 grams per liter of electrolyte, the efiective range being from 3 to 10 grams per liter of electrolyte. Preferably the range is between 3.5 and 7.5 grams per liter of electrolyte. Between 5 and 6 grams densities.

per liter of electrolyte gives excellent results.

It is desired to emphasize the fact that the relatively high temperatures and the relatively high content of sulphate radical coniointly are exceedingly important in securing the results which are obtainable with my process, namely the production of the new type of chromium deposit and the successful use of high current use higher current densities than those in commercial use it was deemed necessary to use the prior art sulphate content, Or actually to lower it. I have discovered that the reverse is true. The conjoint use of higher temperatures and higher sulphate contents is of major importance in working with high current densities if a satisfactory product is to be obtained. 7

As an example of an efiective composition, I may use an electrolyte containing2'l5 grams of chromic acid per liter of electrolyte and 5 /2 grams of sulphuric acid per liter, employing a bath temperature between 58 C. and 85 0., and using a current density of 700 to 7000 amperes per square foot of cathode surface. With this composition, at a temperature of 70 C., a current density may be employed of from 800 to 4060 amperes per square foot of cathode surface, 1500 amperes per square foot of cathode surface being an example. With the same composition, at a temperature of 60 C., a range of 700 to 1600 amperes per square foot of cathode surface may be employed, 900 amperes per square foot of cathode surface being an example. With the same composition, employing a bath temperature of C.. a range of current density of from 2000 to 6500 amperes per square foot of cathode surface may be employed, 4000 amperes per square foot of cathode surface being an example. The current efficiency under theseconditions varies In the abortive prior art attempts to from 15% to 21%, the variation occurring with varying current densities and temperatures.

As another example of bath composition, one may employ an electrolyte containing 185 grams of chromic acid per liter of electrolyte and 3.8

grams of sulphuric acid per liter, using a bath temperature of between 55 C. nd 90 C., and using a current density of from to more than 6000 amperes per square foot of cathode surface. Using a bath of this composition, at a temperature of 65 C., the current density may vary from 500 to 2000 amperes per square foot, 1200 amperes per square foot of cathode surface being an example. With the same bath, at a temperatureof 75 C., the current density may range from 700 to 4200 amperes per square foot of cathode surface, 2200 amperes per square foot of cathode surface being an example. Using the same of chromic acid per liter of electrolyte and 5.5

grams of sulphuric acid per liter. For this bath one may maintain the bath at a temperature of 60 C. to 85 C., employing a current density of from 700 to 10,000 amperes per square foot of cathode surface. With this bath, maintained at 60 C., one may use a current density of 500 to 2000 amperes per square foot, 900 amperes per square foot of cathode surface being an example. With the same bath, at a temperature of 70 C., the current density may vary from 750 to 6000 amperes per square foot. 2000 amperes being an example. The same bath, maintained at 75 C., the current density may vary from 1400 to 7500 amperes per square foot of cathode surface, $009 amperes being an example. Using the same bath, having a bath temperature of 80 C., the current current efliciency from 16% to 26%.

As a further example of bath composition, and conditions to be utilized therewith, may be men-, tioned the use of an electrolyte containing 235 grams of chromic acid and 6.25 gram of sulphuric acid per liter of electrolyte, the bath being maintained at a temperature of 60 C. to 90 C., and a current density of from 500 to more than 8000 es, under these conditions.

amperes per square foot of cathode surface. With the bath at 65 C., the current density may be from 500 to 3000 amperes per square foot, 1000 amperes per square foot being an example. At 75 C., the current density should be from 1000-to 6500 amperes per square foot of cathode surface, 2500 amperes per square foot being an example. At 85 C., the current-density may b from 4000 to more than 8000 amperes per square foot of cathode surface, a specific example being 6000 amperes per square foot.

During the working of a particular operation. it is desirable to maintain the concentrations of .the electrolyte substantially constant with respect to the chromic acid and sulphate concentrations. The concentration of each of these ingredients is preferably maintained constant within a range of about plus or minus 3%. In accomplishing this,

the volume of the solution should be maintained reasonably constant, i. e., to about 2%. In replenishing losses by evaporation it has been comv mon practice to add water from a hose or tap onto density may. vary from 3500 to more than 10,000

amperes per square foot of cathode surface, 6000 amperes being an example. Under these conditions the efficiency varies from 15% to 23%.

Another'example' of bath composition, suitable to my invention, is one containing 185 grams of chromic acid and 5.3 grams of sulphuric acid per liter of electrolyte. This bath should be maintained at a temperature of from 65 C. to 85 C. Under these conditions a current density may be employed of from 500 to 7500 amperes per square foot of cathode surface. Using this bath, at a temperatureof65 C., a current density of from 400 to 2000. amperes per square foot of cathode surface may; be, employed, 1000 amperes per square foot being an example. With the same bath at a temperature of 75 ,C., the current density employed may be from 600 to 4500 amthe surface of the bath. Since the rate of evaporation of my plating bath is more rapid than the baths heretofore used, due to the higher temperature of my operation, it is apparent that there must be a considerable addition of make-up water added in such away as to prevent the composition of the bath in the immediate vicinity of the parts being plated being so altered as to affect the quality of the chromium deposit. I prefer to introduce the water beneath the surface of the solution and uniformly along the length of the tank. This addition may be by a constant small -feed of water so adjusted as to compensate for evaporation losses.

The control of the composition of my baths may be accomplished by analytical means, but in operation such analysis is necessary only at long intervals if a record of the ampere-hours used in a particular tank is kept, so that additions of chromic acid proportional to the chromium deposited may be added. I have found that a factor may readily be determined by which to multiply the theoretical chromi acid addition so as to include also in the replenishment the amount of material lost through drag out and through mist and spray. A preliminary rinse of the plated work over the tank by' using a hose returns a predominant proportion of the solution to the plating electrolyte, which would otherwise be carried away from the plating tank. A baffle or trap may be inserted in the main ventilating duct to cause mist condensation and hence make this source of material loss recoverable. If such refinements are employed, the total chromic acid consumption approaches more nearly to the theoretical and in such a case I have used a factor of 1.2 times the theoretical chromic acid consumption which was derived from the ampere-hours passed through the cell and an estimated average efficiency of portion of sulphate in the-bath will permit the varyingcurrent densities above 1500 amperes per Since such a procedure reduces the rate of drift of the average composition with time to a value which depends on the error of the factor chosen, I have found it seldom necessary to make a chemical analysis of an operating solution.

An important aspect of my invention is the relationship between the emciency and the current density. In the principal range of current density which I use in my processes, for example from 1000 to more than 5000 amperes per square foot, the current eificiency increases uniformly with increase in current density. In other words a graphical portrayal of this relation is a straight line. In the prior art practice, the similar relation is a curved line, and exists for usable tarnish resistant chromium coatings over a narrow and very low range of current density as compared to my conditions. The important behavior of linear correlation between the efficiency of the chromium deposition in my processes and the current density prompted me to discover how the rate of change of the efficiency could be altered and controlled so that the production of chromium plate could be accomplished over a wide range of current density and at speeds much greater than have been achieved before. On examining the quality of the chromium which I obtain in my processes in this way, I found that it possessed unusual new properties including inherent toughness and mechanical stability under static and dynamic loading so that new uses for chromium coated steel and non-ferrous metals were found.

In the priorart practices, which use a mean current density of about 150 amperes per square foot of cathode surface and an electrolyte temperature of 45 C., the efflciency usually varies from 10 to 16% over a range of 100. amperes per square foot of cathode surface, or an increase of 60% per 100 amperes, referred to the lower value.

In my process the variation in eillciency with current density varies to a much less extent. In a typical case. using an electrolyte containing 275 grams of chromic acid and 5.2 grams of sulphuric acid per liter of electrolyte, which is maintained at a temperature of 70 C., the emciency varies from 16% to 22% when the current density increases from 800 to 4000 amperes per square foot of cathode surface, which is a variation of 38% in 3200 amperes or an increase of less than 0.2% per 100 amperes, referred to the lower value. It will be noted that the current efficiency, when utilizing my invention, averages considerably higher than in the prior art, with obvious advantage in speed of deposition and in economical operation. Employing an electrolyte containing 400 grams of chromic acid and 4 grams of sulphuric acid, and maintaining the electrolyte at a temperature in the range 70 C. to 80 C., there is no change in emciency above 2000 amperes per square foot so that the variation of the efliciency may be said to be 0% per 100 amperes. In general, it may be stated that when employing at least 400 grams of chromic acid and at least 3% grams of sulphate per liter of electrolyte, while maintaining the temperature of the electrolyte at a value in the range in excess of 0., the variation of efficiency of chromium deposition with square foot of cathode surface may be made negligible, or negative. I have not observed such effects below the concentration of 3'75 grams per liter of chromic acid. I

This feature of my invention whereby the composition and conditions may, if desired, be adjusted so that small or-negligible variations in efliciency occur with varying current density is of great importance in the distribution of the deposit in that I am able to obtain far greater uniformity of coating than is possible with prior art methods.

This will be readily apparent when the fact is remembered that in electrolytic deposition the current densities frequently vary at different points of the article being plated. On projections of the article, the article of course being the cathode, the current density tends to be high while in depressions the current density tends to be low.

As is well known, the amount of metal deposited at a particular part of an object depends not only upon the current density at that part but also upon the current efficiency. If an object has a greater current density at one site than elsewhere, due, let us say, to a projection at that site. the rate of deposition of metal at the projecting part is greater, not merely because of the effect of the increase in current density, but also because of the increase of the efficiency. As we have indicated above, the conditions of deposition in the processes of the prior art are such that'variation of current density is accompanied by substantial variation of current efiiciency. Consequently, in

' such prior art processes the rate of deposition at projecting portions is considerably more than proportional to the'increase of current density at the projecting part. Also, in depressions 0f the object being plated, where there is a reduction of current density, there is accompanying falling oil. in currentefliciency, resulting in less metal being deposited in the depression not merely because of the lowering of the current densitybut also because of the considerable diminution of emciency. In my process, however, under conditions where there is little or no variation in eniciency with variation of current density, there is but little or no increase or diminution of rate of deposit due to the efllciency factor. Therefore; the deposition is more uniform than in the prior art.

Not only is it possible to operate my process so as to obtain substantially uniform efficiency of deposition with varying current density, but it is also possible to so operate my process as to obtain a gradually lessening eillciency of chromium deposition as the current density is increased.

For example, employing an electrolyte containing 400 grams of chromic acid and 3.50 grams of sulphuric acid per liter, the temperature of the bath being maintained at C., as the current density is increased from 1500 amperes per square foot of cathode surface the current efflciency is gradually reduced. Under the conditions, just indi and 3600 amperes per square foot of cathode sur- I face.

The relationship may be further exemplified shearer by considering the variations of emciency with varying amounts of sulphate in the electrolyte. with an electrolyte containing 400 grams of chromic acid per liter and containing grams of sulphuric acid, maintained at a temperature of 80 0., there is a slight increasein efliciency of chromium deposition on increasing the cathode current density. With the same content of chromic acid, but with the bath containing 4 grams of sulphuric acid per liter, .the efliciency issubstantially constant. With the same bath, maintained at the same temperature, but with the sulphuric acid content reduced to 3.5 grams per liter, the efllciency diminishes as previously indicated. With the same bath, maintained at the same temperature, but with the sulphuric acid reduced to 3 grams per liter, the efliciency diminishes from 20.5% to 9% as the current density is increased from 1500 to 3600 amperes per square foot of cathode surface.

.the reduced efllciency of deposition will act to nullify to a greater or less extent the undue deposition of metal at the projection. In depressions, for example, where the current density. tends to become less, there will be an increase of efllciency of deposition to compensate at least in part for the reduction of current density.

Another important feature of my invention is the control of the trivalent chromium in the electrolyte. When a chromium acid solution is electrolyzed, the chromic acid, or hexavalent chromium, is reduced, 1. e., from CrOa to CraOa, or equivalent forms. Simultaneously at the anode the reverse process occurs, that is the trivalent chromium is oxidized tohexavalent chromium.' Therefore, a dynamic equilibrium tends to be established between the trivalent and hexavalent chromium in the electrolyte. I have discovered that trivalent chromium when present above certain concentrations, which concentrationsvary with the conditions of operations, has a deleterious effect on the properties of the chromium deposits. For thick deposits, say of the order of 0.010 inch, too high a concentration of trivalent chromium reduces the practical upper limit of current density which can be employed, because of roughness and nodulation of the deposit. Furthermore, the deposit becomes more brittle and inherently weaker. Therefore, for

1 optimum operation I prefer to control the trivalent chromium concentration ordinarily between the limits of 0.5 and 5.0 grams per liter of electrolyte. The trivalent chromium should not be allowed to exceed 10 grams per liter, whenever the resulting chromium deposit is to withstand mechanical loading. V

The concentration of trivalent chromium may be regulated in several ways. Several examples will be given here for purposes of illustration.

The formation of trivalent chromium may be favored or retarded by differences in the relative anode and cathodeareas. When the approximate ratio of anode to cathode area is. less than 2,

the relative rates of formation and oxidation of processes. If the ratio is greater trivalent chromium favor an increase in concentration of that component of the bath in my than about 2, the converse is true. This ratio oi 2 is approximate and varies somewhat with the varying conditions of operation.

A simple manner of regulating the trivalent concentration is by varying the type of work going through the bath. For example, if a series of parts, which are to be plated on the internal surface, requiring an anode having a small surface area in comparison with that of the cathode is utilized, such parts could be alternated in suc-' parts having a large cessive operations with cathode area relation. In this way the concentration of trivalent chromium will alternately be increased and reduced, the operation of course being so conducted that the trivalent chromium v is never allowed to rise to the point whereit has go temperature greater than 90 C. and employing f an injurious efiect upon the deposited chromium.

In the practical application of my processes, certain precautions should be taken. As earlier indicated in this specification, the temperature chosen for the electrolytic bath is very important. In practice. it is advisable to prevent the temperature of the bath varying too greatly. Ordinarily it is best to operate at a substantially uniform temperature for any particular operation. The working temperature having been determined upon, it should usually be maintained constant within a range of plus or minus one degree centigrade.

.An easy way of preventing inequalities of temperatures within a plating bath, and to prevent undue localized variations in composition of the bath, is to use vigorous air agitation of the bath during electrolysis.

From a practical standpoint I find it useful, and some times important, to bring the article to be plated, or at least the surface portions thereof, up to the temperature of the plating bath before starting the plating operation. This is important when operating with-objects of substantial mass, such as heavy shafting, for example. This is far more important in my type of plating, which involves the use of relatively high temperature plating baths, than in those plating "operations which are worked with the prior art baths employing relatively low temperature. As with other coating processes, the physical and chemical condition of the surface of the article being plated is important with my methods. It is especially important that the surface of the article to beplated be as clean as possible. There are various treatments to this end which may be employed. For example, when treating steel the article is first degreased with an organic solvent. It is then cleaned cathodically in an alkaline solution greater than pH11, having a a current density greater than 50 amperes per square foot of anode surface. The article is then dipped in 10% sulphuric acid at a temperature of the order of 50 0. for a period greater than ten seconds.

.Another example of a method of preparing steel articles for plating is to clean the same in an alkaline solution, using no electrolysis, and then dip in 10% hydrochloric acid, subject to electrolysis as an ahode in 50% sulphuric acid at room temperature, rinse and then immerse in the chromium plating solution preparator to plating..

A third example of a method or preparing cleaned steel articles for plating with chromium articles electrolytically.

by my processes consists in the step electrolyzing the work to be plated as anode in the chromic acid plating electrolyte at the temperature atwhich the plating is to be accomplished for a period of from to 45 seconds and a current density of from 200 to 500 amperes per square root. As one particular example, in a plating electrolyte operated at 70 C., a current density of 300 amperes per square foot may be used anodicaily on carbon steel for twelve seconds. With such a treatment the adherence oi the subsequently electrodeposited chromium is exceptionally good.

It will be understood that these are merely examples of numerous methods of preparation for coating which may be used.

' A very useful feature or my processes are their applicability for plating on different metals. with the prior art processes it is not easy to get a good bond directly on carbon steel and the difficulty is much greater with alloy steels.

With my processes I have never found-any serious dimculty in plating chromium on any carbon steels, alloy steel, or cast iron. With the processes it is common to first plate with of nickel or copper before plating with purpose of obtaining a coating of chromium.

prior art a coating the chromium for the satisfactorily adherent While a preliminary coating of some other metal such as nickel or copper may be employed in conjunction with my processes, the processes themselves are entirely successful without such preliminary coating. For example, I am able to plate directly on cast irons, such as white cast irons, with my processes and the chromium coating is so adherent that it can be subjected to considerable impact without spalling,

Hardened steel parts can be plated by my processes. For example, I have successfully used these processes for chromium coating on steel articles which were case-hardened and also on those which were nitrided. In neither of these cases is there any diflicult in securing a satisfactory permanent bond between the chromium and the hardened surfaces. When using the prior art processes, rienced with alloy steels. When using my processes this diiiiculty is .avoided. For example, I have plated exhaust valves made of alloy steels containing from 12 to 13% nickel and from 12 to 13% chromium, with other constituents, to

give a chromium coating of more than .010 inch thickness. The adherence between the valve material and. its coating is so great that the valves withstand prolonged service which involves alternate heating and cooling, the maximum temperature of such use being red heat.

Stainless steels of the so-called 18-8. variety have been so diflicult to plate with chromium with prior art processes that it has been necessary to develop special methods for coating these With my processes it is possible to plate chromium directly on stainless steels without making any special provisions because of the nature of the composition of the article being treated. r

The invention described herein ma be manufactured and/or used by or for the Government of the United States of America for governmental purposes without the payment of any royalties 70 thereon or therefor.

Having thus described my invention what I claim as new and desire to secure by Letters Pat-' cut is:

dense, malleable chromium having high resistance to impact, the step of electrolyzing an electrolyte containing chromic acid and containing between 3 and 7 V grams of sulphate radical per liter of electrolyte at a current density of at least-700 amperes per square foot of cathode surface while maintaining the temperature of the electrolyte between 60 and 90 C.

2. In a process for the electrodeposition of hard, dense, malleable chromium having high resistance to impact, the step of electrolyzing an electrolyte containing chromic acid and containing 5 to 6 grams of sulphate radical per liter of electrolyte at a current density of at least 700 amperes per square foot of cathode surface while maintaining the temperature of the electrolyte between 60 and'90" C.

3. In a process for the electrodeposition of hard, dense, malleable chromium having high resistance to impact, the step of electrolyzing an electrolyte containing chromic acid and 5 to 6 grams of sulphate radical at a current density between 700 and 5000 amperes per square foot of cathode surface while maintaining the temperature of the electrolyte betwen 60 and 90 C.

4. In a process for the electrodeposition of hard, dense, malleable chromium having high resistance to impact, the step of electrolyzing an electrolyte containing chromic acid between 150 and 600 grams per liter and between 3 and 7 /2 grams of sulphuric acid per liter of electrolyte at a current density between 700 and 5000 amperes per square ,foot of cathode surface while maintaining the dense, malleable chromium having a high resistdifficulty is frequently expeance to impact, the step of electrolyzing an electrolyte containing chromic acid between 350 to 600 grams per liter and 5 to 6 grams of sulphuric acid per liter of electrolyte at a current density between 800. and 3000 amperes per square foot of cathode surface while maintaining the temperature of the electrolyte between 60 and C.

6. In a process for the electrodepositicn of hard,

dense, malleable chromium having a. high resistance to impact, the step of electrolyzing an electroh'te containing more than 3% grams but not more than 7.5 grams of sulphate radical and more than 3'75 grams of chromic acid per liter oi electrolyte while maintaining the temperature of the electrolyte greater than 60 C. and the current density at least 1000 amperes per square foot of cathode surface, to minimize the variations of efficiency of chromium deposition with varying current density.

7. In a process for the electrodepositlon of hard, dense, malleable chromium having high resistance. to impact, the step of electrolyzing an electrolyte containing chromic acid and containing at least 5 grams but not more than 7.5 grams of sulphate radical per liter of electrolyte at a current density of at least 700 amperes per square foot of cathode surface while maintaining the temperature of the electrolyte at as high as 55 C.

8. In a process for the electrodeposition of hard, dense, malleable chromium having high resistance to impact, the step of electrolyzing an electrolyte containing between 150 and 600 grams of chromic acid per liter of electrolyte and containing between 3.5 and .10 grams of sulphate radical per liter of electrolyte, at a current density of at least *1. Inaprocess iorthe electrodeposition of hard, is amp s p r Square foot of cathode u while maintaining the temperature of the electrolyte between 55 C. and 90 C.

with decreasing current 10. In a process for the electrodeposition of hard, dense, malleable chromium having high resistance to impact, the step of electrolyzing an electrolyte containing at least 3.00 grams but not over about 4 grams of sulphate radical and more than 375 grams ofchromic acid per liter of elec-' troiyte, while maintaining the temperature of the electrolyte greater than 60 C. and the current density at least 1000 amperes per square foot of cathode surface to effect a reduction of the current emciency with the increasing current density and to effect an increase of the current efliciency density.

RICHARD M. WICK. 

